PSI has been shown to adequately reflect the associated heat strain for different climatic conditions, types of clothing including protective clothing, various hydration levels, different exercise intensities, gender, and the effects of aging. In addition, adjusted PSI can discriminate the heat strain in non-human species such as rats during exercise-heat stress. Thus, PSI does not seem to require any significant refinements. CSI has been shown to be effective in depicting cold strain for both cold air and cold-water immersion during resting conditions. However, CSI may need to be further adjusted to better consider the effects of physical exercise, and also a wider range of cold air and water temperatures. We speculate that the addition of HR to CSI as a third component to this index could potentially be the proper adjustment. Nevertheless, both indices have the potential to be widely accepted and used universally for many military scenarios.